Bladders or bellows are well known commercial devices for controlling the relative movement between two parts. These devices typically include a guide and upper and lower limit stops. U.S. Pat. No. 1,169,250 to Fulton discloses a shock absorber for water pipes. The device has a collapsible and expandable vessel located between and secured to centrally perforated inflexible end walls. The apparatus includes a guide for limiting relative lateral motion of the plates. U.S. Pat. No. 1,928,368 to Coffey pertains to vehicle jack with three telescoping cylindrical sections. A collapsible and extendable rubber sack or lining is used inside the telescoping sections. Shoulders and flanges limit outward telescoping movement. U.S. Pat. No. 3,935,795 to Hawley discloses a bellows actuator with opposed ends that are closed by circular disks. The disks are mounted to hubs that are rigidly secured to a shaft. The hubs limit the minimum and maximum length of extension. U.S. Pat. No. 4,292,885 to Jinnouchi pertains to an apparatus having a bellows with a main body and a restriction means. The restriction means restricts elongation and contraction via guide metals, guide members and stoppers. Jinnouchi recognizes the limited stroke length (SL) associated with telescoping sleeves.
Bladder actuators are well known in the railroad industry. In 1882, the Smith Vacuum Brake included a sack or collapsing cylinder. The sack has upper and lower plates and a flexible bladder joined to the generally round perimeter of the plates to form an air-tight seal. The sack includes an internal guide mechanism formed by a guide sleeve and guide rod positioned along a centerline of the sack. The pinned connections allow the guide sleeve and rod to guide the motion of the sack. The sleeve is pinned to the frame of the railroad car, passes through the upper plate and extends into the sack a given distance. The guide rod is coupled to the lower plate, pinned to a braking assembly and is slidingly received in the larger diameter sleeve. When vacuum is supplied to the sack, the lower plate and guide rod are drawn up, which moves the brake assembly to a braking position. The engagement of the brake pads against the wheels of the train forms an upper stop for the sack. When vacuum is relieved, the lower plate and guide rod of the sacks are biased to drop down under the weight of the brake assembly, which moves the brake assembly to a non-braking position.
The Firestone AIRSTROKE actuator developed in the 1930s includes upper and lower plates and a flexible bladder secured around the perimeter of each plate to form an airtight interior. The actuator is inflated and deflated to control its height. Down and up stops are used to set the minimum and maximum height or stroke length (SL) of the actuator. A bumper, a chain, a cable or metal stops can be located inside the actuator for this purpose. Firestone recommends guiding the stroke of the actuator. The actuator is recommended for use in a wide variety of applications including braking applications, such as a Roller Friction Brake with an external guide rod and guide sleeve with upper and lower stops. Firestone acknowledges that companies such as Selson and Minster have modified the actuator to locate all the guide and limit stops inside the actuator. The AIRSTROKE actuator has been used in railroad braking systems. U.S. Reissue Pat. No. Re 33,207 to Brodeur discloses an on-board braking system using the Firestone actuator. U.S. Pat. No. 6,220,400 to Kickbush discloses a low profile, railway car retarder using the Firestone actuator. The actuator has an internal guide formed by two telescoping tubes, one of which has a stop ring at its end to form the upper and lower limit stops.
The railroad marshalling yard environment is dirty, rugged and non-stop. Retarders, switches, actuators, compressed air controls and other components along tracks must withstand exposure to harsh weather, dirt, gravel, petroleum and other chemicals, and withstand being struck by moving objects carried by the cars. Moreover, actuators for retarders produce static vertical forces of about 20,000 pounds to generate the necessary braking power to control the speed of a fully loaded railroad car. Given this demanding environment, the railroad industry places great significance on minimizing maintenance and down time. Bladder actuators must withstand large cyclical loads and a harsh environment while maintaining low maintenance and down time requirements similar to conventional rigid cylinder actuators. For safety reasons, the guide mechanism and limit stops of the bladder actuator are preferably located inside the actuator to minimize the chance of a worker inadvertently getting his or her fingers caught between the moving parts when the actuator is rapidly opened or closed. Given that bladder actuators are typically round, the obvious location of an internal guide and limit stops is toward the center of the actuator.
A problem with an internal guiding mechanism for a bladder actuator is the rapid wear of the internal friction bearing. The actuator produces about 20,000 pounds of upward force to move the plates apart. The friction bearings also experience lateral loads of over 1,000 pounds to maintain the upper and lower plates in parallel alignment and concentric registry. The concentric, telescoping guide rod and guide sleeve include a friction bearing or bushing to allow sliding engagement as the actuator opens or closes. Accelerated wear of the bushing occurs when the lateral loads push guide rod out of concentric alignment. Deflection of the guide rod causes an exponential increase in the lateral load, which increases the frictional forces and wear on the bushing. The worn bushing allows further misalignment of the guide rod, increased lateral loads, and even more rapid wear of the busing. This is a particularly significant problem with actuators for low profile retarders because a short bushing length is not able to distribute the lateral load over a large busing surface area. The ends of the bushing tend to wear quickly. Yet, frequent maintenance to replace the bushing is time consuming and expensive and results in costly down-time for the yard.
Another problem occurs when an internal guide rod forms the upper and lower stops of the actuator as in U.S. Pat. No. 6,220,400. The guide rod experiences a tension load in excess of 20,000 pounds each time the actuator is opened. This cyclical load loosens the threaded engagement of the guide rod to the upper plate. Yet, as noted above, maintaining the alignment of the guide rod is critical. Even a slight loosening of the guide rod can result in some lateral movement, which will exponentially increase the loads on and wear rate of the internal bushing or bearing. This loosening of the guide rod, or even the potential loosening of the guide rod, significantly increases the need for routine maintenance and possible down time.
A further problem with an internal guide mechanism for bladder actuator for a low-profile railroad retarder is the trade off between stroke length (SL) and bushing length. A certain amount of stroke length (SL) is necessary given the geometry of the retarder and its levers. The actuator must ensure that the brake pads come together close enough to ensure that proper braking force is applied to the wheels of various railroad cars. The actuator must also ensure that the brake pads retract sufficiently far from the railroad car wheels when in a non-braking position. Inadvertent contact with the wheels can result in derailments and loss of life. Yet, as indicated in Jinnouchi, when a guide mechanism is fixed entirely between the upper and lower plates and uses a stop ring at the end of the guide rod, the maximum stroke length (SL) is ½ the distance between the plates when the actuator is in its full open position. The stroke length is further reduced by the length of the bushing engaging the guide rod. Thus, an actuator for a low profile retarder as in FIG. 3 of U.S. Pat. No. 6,220,400, the length of the bushing is kept to a minimum in order to reduce the height of the actuator and obtain necessary stroke length (SL). Yet, a short bushing will have difficulty maintaining concentric alignment of the guide rod and will wear quickly and require frequent maintenance.
The present invention is intended to solve these and other problems.